T7 RNA polymerase is a a single sub-unit DNA dependent RNA polymerase from the T7 bacteriophage. Like most RNA polymerase T7 RNA polymerase transcribes DNA through three phases
-Initiation
-Elongation
-Termination

The initiation phase continues until the enzyme synthesizes ~8 base RNA after which it loses promoter contacts and transitions into the stable elongation phase. The initiation phase in T7 RNA polymerase like all other single and multi subunit RNA polymerases is characterized by inherent instability leading to the release of small RNA (3-8 bases long). In addition to the crystal structure of the enzyme with a 3 base DNA-RNA hybrid (PDB ID:1QLN) this 7 base intermediate structure provides a insight into the structural changes within the enzyme has it synthesizes RNA during initiation and the mechanism which eventually leads to the release of the promoter contacts and the transition into elongation.

In the image below, the template strand (blue) has been separated from the non-template strand (green) and forms a 7 base hybrid with the RNA (red).